Before 1850, U.S. consumption of wheat was lower than between 1850 and 1910. The USDA explains:
"Wheat production was difficult in New England and in much of the South in the colonial era (1600s and 1700s), making wheat flour too expensive for regular use. High transportation costs also made long-distance transport of wheat and flour from regions better suited for wheat growing unprofitable. Therefore, colonists in these regions turned to other crops, especially corn. The wealthy were the principal consumers of wheat bread."
In the U.S., wheat flour (and thus wheat bread) consumption peaked in the late 19th century at about 220 pound per capita per annum. That means that in the late 19th century, U.S. citizens were consuming an average of about 10 ounces of wheat flour daily.
That would provide about 950 kcalories, 37 g protein, and 200 g carbohydrate from whole wheat flour alone. It is equivalent to consuming 12 slices of whole wheat bread daily.
About 1910 the per capita wheat flour consumption dropped below 200 pounds, and now it is about 150 pounds per annum, so we have seen a 32 percent drop in wheat flour consumption since the late 19th century.
So, during the 20th century, what replaced grain consumption? The USDA says :
"Historically, economic development has been accompanied by the substitution of meat for grain in the diet, and this was true in the United States starting in the 1870s."
The substitution of meat for grains as a consequence of economic development represents part of what nutritionists have called the nutrition transition , which Popkin characterizes here :
"Major dietary change includes a large increase in the consumption of fat and added sugar in the diet, often a marked increase in animal food products contrasted with a fall in total cereal intake and fiber."Put another way, up until the late 19th century, the U.S. was an agrarian nation. Agrarian nations typically derive most of their sustenance from cereal grains and this was true of the U.S. for its first 100 years, during which corn and wheat provided the majority of calories consumed by the majority of people.
So far as I can tell, rates of obesity, diabetes, and cardiovascular disease in the U.S. went up during a time when consumption of wheat and corn was well below that of the late 19th century.
According to an article by B.C. Curtis posted on the Food and Agriculture Organization website, the French consume almost twice as much wheat per capita as people of the United States.
According to World Health Organization data, the U.S. has about 2.5 times as many heart disease deaths as France.
France also has an obesity rate one-third of that of the U.S.A..
It doesn't look like consumption of bread increases the risk of heart disease or obesity in France.
We do have evidence indicating that whole grains including whole wheat protect against heart disease.
Groen (pdf) studied the effect of dietary wheat bread on serum cholesterol. He found first that Trappist monks, Yemenite Jews, and Arab Bedouins consumed an average of 600, 500, and 750 g of bread daily, compared to an average of 150 g bread daily in a Western diet, but the bread eaters had low serum cholesterol levels and very low risk of ischemic heart disease.
The Bedouins consumed the most bread (750 g daily) and the least animal protein (5 g daily) and total fat (38 g) and had the lowest serum cholesterol.
One slice of bread weighs about 30 g, so the Trappists ate about 20 slices daily, the Yemenites about 17 slices, and the Bedoins about 25 slices daily, compared to about 5 slices in the typical Western diet. These levels of bread intake are common among people who eat bread as a staple food.
Groen compared the effects on serum cholesterol of low-fat, low-sugar diets in which most of the protein came from animal sources, or most of the protein came from wheat gluten. The study suggested that a gluten-rich diet may produce a lower cholesterol level than one based on animal protein.
Groen also found that replacing bread with equal caloric amounts of sugar raised serum cholesterol.